1. Field of the Invention
The present invention relates generally to a method of making a cantilever stylus for use in an atomic force microscope, and more particularly to a method of making a flat-cantilever having a free end, of which an edge is used as a stylus.
2. Description of the Prior Art
Conventionally, a scanning tunneling microscope (hereinafter referred to simply as STM) has been developed as an instrument capable of observing the surface of a solid body on the atomic scale. However, since the STM detects a tunnel current between a sample and a stylus to observe the surface of the sample, it is impossible for the STM to observe the surface of an insulator. In order to solve this problem, an atomic force microscope (hereinafter referred to simply as AFM) for observing the surface of a sample by detecting forces acting between the sample and the stylus has been proposed. Because the AFM detects minute forces, it requires a cantilever having a stylus and a length of about 100 .mu.m. The resolving power of the AFM depends greatly upon the radius of curvature of the stylus. The less the radius of curvature is, the higher is the resolving power.
FIG. 1 depicts a conventional cantilever 1 for use in an atomic force microscope. This cantilever 1 is a flat-cantilever formed of a thin film of SiO.sub.2 or Si.sub.3 N.sub.4. An edge 2 of the cantilever 1 is used as a stylus. This kind of cantilever is generally made by the use of the photolithography technique.
Because a cantilever for an atomic force microscope generally has a length of about 100 .mu.m, the thickness thereof must range from 1 to 1.5 .mu.m in consideration of strength thereof. In making such a cantilever, etching must be performed with respect to a thin film having a thickness of about 1-1.5 .mu.m. In an ordinary photolithography process, after a photoresist has been formed on a thin film of cantilever material such as, for example, SiO.sub.2 or Si.sub.3 N.sub.4, etching is performed with respect to the thin film with only a portion thereof covered with the photoresist left. The wet-etching technique is generally employed in this etching and the dry-etching technique is not available because the film of cantilever material is relatively thick for the dry-etching. Even by the use of the wet-etching technique, however, it is likely that such a thickness would cause separation of the photoresist during etching, thereby causing a relatively large under-etching in which that portion of the film of cantilever material from which the photoresist has been separated is excessively etched. As a result, an edge of the cantilever made in the above-described manner has a radius of curvature of about 1 .mu.m in a direction parallel to the surface of the cantilever and a radius of curvature greater than it in a direction perpendicular to the surface of the cantilever. An atomic force microscope employing such a cantilever cannot possibly obtain a resolving power on the atomic scale.